The present invention relates to a video coding device for compressively encoding a video signal by reducing redundant data contained therein and a video decoding device for decoding the compressed coded video signal and, more particularly, relates to a video coding device for hierarchically encoding a video sequence by previously performing motion-compensated interframe prediction of video frames, dividing an obtained prediction error signal into frequency-band-components and separately encoding each of the frequency-band-components and a video decoding device for decoding video signals coded by the video coding device.
With recent developments of multimedia services, the image processing technique gets an increasing importance and various kinds of studies concerning to the image processing have been made in various fields of applications.
In general, video information contains a very large amount of information. Accordingly, it is impossible to practically transmit video information as it is, because of necessity of a transmission line with a very wide-band transmission capacity. On the other hand, a video signal contains redundancy to be reduced. Therefore, a method for encoding a video sequence by reducing redundant data contained therein is widely applied as a compression coding technique to treat video signals at high efficiency.
There's an exemplified structure of a prior art video coding device using an interframe-prediction orthogonal-transform coding method. In the prior art video coding device a motion-compensated interframe-predicting portion encodes input video signals per frame by motion-compensated interframe-predicting method and outputs motion-vectors. Namely, a preceding video frame image which has been coded, decoded and then stored in a frame memory portion is read by the motion-compensated interframe-predicting portion as a predicted video signal. A difference calculating portion determines a prediction error signal that is a difference between the input video signal and the predicted video signal read from the frame memory portion, thus eliminating temporal redundancy from the signal to be encoded.
The prediction error signal outputted from the difference calculating portion is transferred to an orthogonal transforming portion whereby the signal is orthogonally transformed and removed off spatial redundancy. In consequence of this processing, a transform coefficient is outputted.
The transform coefficient from the orthogonal transforming portion is quantized and encoded with a compressed amount of information by a coding portion.
The output signal from the coding portion is transferred as coded information to an external circuit and a decoding portion.
The decoding portion performs processing operations reverse to those performed by the coding portion and outputs the transform coefficient to an inverse orthogonal transforming portion that in turn conducts inverse orthogonal transform of the received transform coefficient.
The output signal from the inverse orthogonal transforming portion is added by an adding portion to the predicted video signal read from the frame memory portion. The resultant signal is stored in the frame memory portion and will be used for interframe prediction of a next input video signal. Generally, this operation is called "Loop-back".
Input video signals are thus encoded by a coding-decoding loop configuration (coding loop).
As described above, the prior art video coding device is capable of efficiently encoding video sequence by previously eliminating temporal redundancy of image information through motion-compensated prediction and spatial redundancy through orthogonal transform.
However, the prior art coding method encodes a video sequence frame by frame and, therefore, a whole image may not correctly be decoded from image information encoded by the conventional coding method if any image signal should have a transmission error or loss of information, which occurred in a transmission line.
Furthermore, the prior art method uses the interframe prediction coding technique and therefore involves a problem of handing down an incorrectness occurred in a decoded image to all subsequent frames to be encoded.
To solve the above-mentioned problems, Japanese Laid-Open Patent Publication No. 9-70047 discloses a hierarchical coding method that arranges image signals in a hierarchy and encodes signals of each hierarchical layer separately from each other. Namely, when incorrect decoding occurred in a hierarchical layer of image signals, this method can minimize the impairment of decoded images by only recoding signals of other correct layers.
There's illustrates an exemplified construction of a hierarchical video coding device using divided frequency bands. Similarly to the above-described prior art method, the device encodes an input video-sequence frame by frame by a motion-compensated interframe-predicting portion and outputs a prediction error signal per frame from a difference calculating portion.
The prediction-error signal from the difference calculating portion is divided into a plurality of frequency bands and then outputted in bands respectively by a band-dividing portion. The band-components of the prediction error signal, which were outputted from the band-dividing portion, are encoded separately from one another by the coding portions respectively. Encoded information a-n is outputted to an external circuit and corresponding decoding portions.
The decoding portions performs processing operations reverse to those performed by the coding portions from which respective band components of the prediction-error signal are outputted.
On the other hand, a predicted image signal read from a frame memory is divided into frequency bands and outputted as respective frequency-band-components by a band dividing portion.
The frequency-band-components of the predicted image signal from the band dividing portion are added to the corresponding frequency-band-components of the prediction-error signal from the decoding portions by an adding portions respectively. The obtained frequency-band-components of a decoded image signal are outputted respectively.
These frequency-band-components of the decoded image signal are synthesized by a synthesizing portion to form a decoded image signal through processing operations reverse to those performed by the band dividing portion. The decoded image signal is stored in the frame memory portion and will be used for interframe prediction of a next input image signal. According to the above-mentioned processing method, it is possible to minimize degradation of a video sequence due to an error or a loss in information transmitted over a transmission line since a decoding error occurred in a certain frequency-band-component may be confined in said frequency band.
However, when transmission error and/or a loss of information occurs in a transmission line, the above-mentioned prior art hierarchical coding device can enclose the decoding error within a hierarchical layer but cannot completely prevent said error from propagating to subsequent images.